Defect and interface engineering of two-dimensional materials for electronic and optoelectronic applications

Abstract

Two-dimensional (2D) materials have attracted significant attention because of their unique electronic structures, outstanding electrical and optical properties. These properties enable 2D materials to be potential materials for electronic and optoelectronic applications. Among 2D materials, 2D transition metal dichalcogenides (TMDCs) are currently promising materials altering conventional Si electronic and optoelectronic devices due to reasonable carrier mobility, intrinsic band gap, transparency, and flexibility. In order to realize the practical electronic and optoelectronic applications, modulating properties of devices is necessary. However, it is challenging to control and improve properties of devices due to Fermi level pinning, which diminishes their performance.

Defect and interface engineering is a crucial approach to improving and modulating electrical and optoelectrical properties of 2D TMDCs; however, understanding how to control defects and interfaces is still lacking.

In this dissertation, defect and interface engineering is developed to improve and modulate electrical and optoelectrical properties of 2D TMDCs: (1) hydrazine treatments are employed to induce S vacancies leading to n-doping effects in WS2 and MoS2, (2) deep-ultraviolet (DUV) light is irradiated into MoS2 to form S vacancies, (3) defect-induced contact properties of MoS2 devices are investigated by observing an interface between metal contacts and MoS2, and (4) graphene/Ag contacts are used to improve electrical and optoelectrical properties of MoS2 devices.

In the first part of this dissertation, I present a simple and facile route to reversibly modulate electrical and optoelectrical properties of WS2 and MoS2 via hydrazine and sulfur annealing treatments. Hydrazine treatment of WS2 and MoS¬2 improves the field-effect mobilities, on/off current ratios, and photoresponsivities of the devices. It is attributed to the surface charge transfer doping and the S vacancies formed by its reduction, which result in the n-type doping effect. The changes in the electrical and optoelectrical properties are completely recovered by the sulfur annealing process. Furthermore, the deep-ultraviolet (DUV) light is introduced to form S vacancies in MoS2. When the DUV light is irradiated into MoS2 under N2 atmosphere, S vacancies are formed due to a higher energy of the DUV than the formation energy of S vacancies. In the case of irradiating into MoS2 under O2-contained N2 atmosphere, oxygen atoms are substitutionary doped on S vacancies formed by the DUV light due to the dissociation with separated oxygen atoms by the DUV light. The control of doping types depending on DUV irradiation atmosphere conditions allows for a change of electrical and optoelectrical properties of MoS2 devices.

In the second part, contact and interface properties of MoS2 devices are explored using various contact metals, such as Ti/Pt, Ti/Au, Ti, and Ag. Ag contacted devices show particularly high performance, achieving the field-effect mobility (12.2 cm2 V–1 s–1), on/off current ratio (7 × 107), and photoresponsivity (1020 A W–1) due to lowering the Schottky barrier height of devices induced by the interface reaction between defects in MoS2 and Ag contacts. Furthermore, a graphene monolayer film is inserted between Ag contacts and MoS2 to further improve electrical and optoelectrical properties. The devices with graphene/Ag contacts achieve the field-effect mobility of 35 cm2 V–1 s–1, on/off current ratio of 4 × 108, and photoresponsivity of 2160 A W–1, which are superior than those of back-gated CVD-grown MoS2 monolayer-based devices reported in previous studies. These improvements are attributed to the low work function of Ag and the tuning of graphene Fermi level; the n-doping of Ag in graphene decreases its Fermi level, thereby reducing the Schottky barrier height and contact resistance between the MoS2 and electrodes.